This invention relates to an integrated gas turbine power generation system and process. It is more particularly concerned with an integrated power generation system wherein carbonaceous fuel is burned in a non-pressurized circulating fluidized bed combustor to provide heated gas for the gas turbine cycle and steam for the steam turbine cycle.
Conventional coal-fired power plants generating electrical power can produce thermal efficiencies of about 37-38% at the most, when the costs of current requirements for clean flue gases are taken into account. With integrated coal-fired gas turbine power plants, higher thermal efficiencies can be achieved. Processes with efficiencies as high as 40-41% as well as with low NO.sub.X and SO.sub.2 emissions are well known.
A typical integrated power plant system includes a pressurized coal combustion system. A compressor provides pressurized air, 6-30 bar, into the combustor. The gas produced in the combustor is cleaned and employed to drive an expansion turbine. A waste heat boiler can be arranged for heat recovery after the turbine. The combustor includes a conventional steam generator to produce steam for driving a steam turbine electrical generator assembly.
The integrated power plants including pressurized coal combustion systems require an expensive and complicated construction. The coal feeding system is not easily arranged. The use of gas turbines is limited by the inability of the gas turbines to accept effluents from lower graded fuels. The high temperature effluents require complicated clean-up systems which increase costs. The turbine blades erode and foul as they are exposed to particulate matter not separated in the clean-up system.
Integrated plants which employ substantially atmospheric coal combustion systems are also known, such as those described in patent publications U.S. Pat. No. 4,326,373; U.S. Pat No. 4,406,128 and WO 82/00701.
U.S. Pat. No. 4,326,373 discloses an atmospheric coal combustion process in which the heat generated is recovered by air compressed in a hydraulic compressor and passed through air heaters (2) in the flue gas passage above the combustion chamber and through air heaters (4 in FIG. 1, U.S. Pat. No. 4,326,373) in the combustion chamber. The compressed and heated air is used for the production of work in a gas turbine. A heat exchanger (4) is exposed to extremely high corrosion/erosion. A heat exchanger (2) will be large and expensive due to inefficient heat transfer between flue gases and preheated air inside the tubes. Heat exchange rates between suspensions and tubes inserted in the suspension are dependent on the particle density of the suspension. In a fluidized bed, there are great variations in heat exchange at different heights in a fluidized bed combustion chamber. Maximum heat exchange can be achieved only in the lower part of the combustion chamber, in the dense zone of the bed.
The air heat recovery arrangements in the combustor can also easily cause disturbances in the combustion process and the overall process will be difficult to control. The temperature profile in the combustor will be affected by these extra heat exchange tubes disposed in the reactor. And furthermore as there already are heat transfer tubes for steam generation arranged in the reactor, it can be difficult to fit in any other tubes.
U.S. Pat. No. 4,406,128 discloses a combined cycle power plant utilizing a gas turbine for generating electric power. Carbonaceous fuel is burned in a non-pressurized fluidized bed combustor to provide energy for driving the gas turbine and generating steam. An external heat transfer unit through which a portion of the hot solids of the fluidized bed is circulated is used to heat clean compressed air. The heated compressed air is expanded in a gas turbine for the generation of electric power in a generator coupled thereto. The external heat transfer unit consists of an air heater chamber with inserted parallel tubes. A portion of hot solid particles from the fluidized bed is conveyed through these tubes with the aid of some fluidizing air. The clean compressed air passes into the air heater and around the outside surfaces of the tubes, extracting heat from them. The construction with an external heat transfer unit is liable to become space consuming and expensive. A uniform distribution of solid particles through the tubes as shown in FIG. 2 (U.S. Pat. No. 4,406,128) is extremely difficult due to enclosed construction as shown in FIG. 2, low volume rate of solid particles and large number of tubes required for the heat transfer. Furthermore, the air used to convey the solid particles will cool the solid mixture and reduce the temperature gradient between the solids and air on the outside surfaces of the tubes.
The use of additional fluidizing air in the air heater for conveying solid particles through the tubes and back into the combustion chamber has a negative effect on the overall performance of the combustor due to the power required to convey the solid particles. Surplus air or oxygen is not wanted into the combustor, neither into the air heater. In the air heater, air could cause carbonaceous solid particles to burn and so increase the temperature over the melting temperature of some of the solid particles and cause agglomeration of particles in the air heater with detrimental effects of the process. Sticky particles could also cause clogging of the tubes.
WO No. 82/00701 discloses a power generation plant with a recirculating fluidized bed furnace operating at a fluidization velocity of 10 m/s and delivering combustion products to a separating section. The solid particles from the separating section are led through a weir chamber to a second shallow fluidized bed. An air heater is disposed in the second fluidized bed space as well as tubes for steam generation, both tubes in different parts of the bed. The bed is operating at a lower fluidization velocity, 0.5 m/s, as is the bed in the furnace. The heated compressed air is supplied partly to a coal devolatilizer in the power generation system and partly to a burner which is connected to a gas turbine for generation of electrical power. This system has the same drawbacks as the previous one, the construction with a second fluidized bed is space-consuming and expensive. The additional air needed in the system to fluidize the second fluidized bed is a drawback and makes it more difficult to control the combustion process and hence the whole system. Compressed air needed to fluidize the bed is of course a cost in itself.
The use of a shallow fluidized bed is not advantageous as the hot fine ash particles tend to flow towards the top of the fluidized bed and the colder particles stay at the bottom of the bed. Consequently, the heat transfer in the bed deteriorates depending on the temperature profile developed in the bed.
It is an object of the present invention to provide an improved integrated gas turbine power generation system, utilizing a non-pressurized fluidized bed combustor to heat clean gas to the gas turbine without the drawbacks in previously mentioned power systems. Another object of the present invention is to provide an integrated gas turbine power generation system which is compact.
It is further an object of the present invention to provide an integrated gas turbine power generation process which has improved efficiency and is reliable and easy to control.
It is also an object of the present invention to provide a method of controlling the combustion temperature in the combustion chamber of a fluidized bed reactor.
According to the invention, there is provided an integrated gas turbine power generation system comprising a circulating fluidized bed combustor for the combustion of carbonaceous fuel at nearly an atmospheric pressure, and steam and gas turbine cycles. The combustor includes a combustion chamber section, a solid particle separator and a vertical return channel for conveying separated solid particles from the particle separator to the combustion chamber. The vertical return channel is connected with its upper end to the lower part of the particle separator and with its lower end to the inlet for solid recycled particles in the combustion chamber. The gas turbine cycle includes a gas compressor for compressing a flow of oxidizing gas, a heat transfer means connected through a duct to the compressor for heating the flow of compressed oxidizing gas, a gas expansion turbine connected through a second duct to the heat transfer means for power generation from the heated compressed oxidizing gas, and a third duct for leading expanded oxidizing gas from the gas turbine to the bottom of the combustion chamber. The heat transfer means for heating oxidizing gas includes heat exchange passages or tubes disposed inside the vertical return channel for leading compressed gas inside the passages or tubes in indirect contact with a relatively dense suspension of separated particles flowing evenly downwards outside the passages or tubes from the upper part of the channel to the lower part thereof. A process for generating power in such an integrated power system is also disclosed.
The present invention offers a compact circulating fluidized bed combustor with an air heater integrated into the circulation of solid material. The heat exchange passages or tubes are disposed in the vertical return channel where the solids density and consequently heat exchange rate is high. Due to high heat transfer rates, the heat exchanger can be built compact. Heat exchange in the vertical channel is uniform. By leading the solid material through the channel by gravitation, no auxiliary fluidizing air is needed differently from prior art fluidized heat exchangers. Therefore, the present invention offers a more uniform heat transfer process throughout the whole heat exchanger, without temperature profiles as in fluidized heat exchangers. The overall efficiency is increased when no pressurized gas/air is needed for the fluidization of hot particles. Besides production costs, fluidized air can have detrimental effects on hot particles such as burning or agglomeration of particles.
The invention makes it possible to control the temperature in the combustion chamber by controlling the temperature of recycled hot particles. The heat transfer from the hot particles in the vertical to the compressed air is affected by changes in the air flow or air pressure.
The major advantages of the invention compared with the conventional cogeneration system are higher power efficiency and potential to controlling the combustion temperature by extracting energy from the recirculation loop as the air used as working fluid in the gas turbine process also acts as combustion air in the boiler. The air flow rate should be controlled to correspond to the boiler load, i.e., 30-100%. The utilization of air as working fluid in the gas turbine process is simple and reliable, no gas cleaning is needed.
As steam generation tubes are already fully disposed in the boiler, it is advantageous to be able to control the temperature profile of the boiler without still trying to increase the amount of heat transfer means in the boiler itself.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.